1. Technical Field of the Invention
The present invention relates to a subscriber line interface in a telecommunications exchange and, in particular, to the provision of subscriber line access to remote subscribers.
2. Description of Related Art
During the initial introduction of telecommunications exchanges, all telecommunications subscribers were connected with their servicing telecommunications exchanges via individual copper wire lines. Each individual conversation would accordingly be carried on a separate pair of wires or set of wires. However, due to the rapid increase in the number of telecommunications subscribers, the number of telecommunications lines connecting to each telecommunications exchange became numerous and unmanageable. This exclusive connection configuration between a single subscriber and its servicing telecommunications exchange was also wasteful since no subscriber used his telecommunications line continuously.
The foregoing problems and concerns were alleviated to some degree by the advent of Pulse Code Modulation (PCM) technology. Using the PCM technology, a multiplexing device, such as a D4 channel bank developed by Bell Labs, was introduced to convert the analog signals of 24 subscriber lines into digital signals for communication over a single communications link. On the other end of the communications link, another D4 channel bank was used to reconvert the digital signals to analog signals for input to the servicing telecommunications exchange. Such an implementation is also known as Digital Loop Carrier (DLC) technology. As a result, a pair of D4 channel banks could handle 24 subscriber lines via a single digital link.
With the successful implementation of the PCM technology, other types of multiplexers known as Subscriber Loop Carriers were introduced to handle greater subscriber line capacity. For example, one of the most prevalently used multiplexers was the Subscriber Loop Carrier (SLC) 96 system developed by AT&T. After the introduction, SLC-96 rapidly became the market standard for implementing the DLC technology.
A SLC-96 serves 96 subscriber lines using only four communications links, with each communication link servicing 24 subscriber lines. On the subscriber side, a SLC-96 remote terminal (RT) serves up to 96 subscribers. On the telecommunications exchange side of the communications links, a Central Office Terminal (COT) is used to connect to the SLC-96 RT and to reconvert the digital signals back to analog signals. The converted data are then input into the servicing telecommunications exchange. Such a connection arrangement using a COT and RT is commonly referred to as an Universal Digital Loop Carrier (UDLC) since the connection between the COT and the RT is independent of any telecommunications exchange. The use of SLC-96 became very dominant, especially in North America, because even the smallest neighborhood almost always had more than 96 subscribers or homes which could be serviced by a single SLC-96 RT device. As a result, each SLC-96 RT could be utilized to its maximum capacity while only requiring four communications links.
Because the above COT equipment for the telecommunications exchange side of the SLC-96 connection is expensive, the Bell Operating Companies (Bellcore) mandated AT&T to standardize the interface part of the UDLC connection to enable each telecommunications exchange to directly implement its own interface module. Such implementation within each telecommunications exchange would enable direct interface with a SLC-96 RT without connecting through an expensive piece of COT equipment. This interface standard became known as Bellcore's TR-TSY-000008 standard, or TR-8 standard, and is incorporated by reference herein. Consequently, each telecommunications exchange was equipped with an interface module (hereinafter referred to as a TR-8 interface module) to connect the digital communications links directly from a TR-8 compliant SLC-96 RT (also known as a TR-8 compatible RT, collectively referred to as a TR-8 RT hereinafter) . An example of such an interface module is an Extension Switch Module (ESM) manufactured by Telefonaketiebolaget LM Ericsson (publ).
With the de-monopolization of the telecommunications industry, a number of different telephone service companies now provide telephone service within the same geographic area. Small telephone service providers became attractive especially for small businesses and building owners because of their price competitiveness. However, due to the size and business environment of their customers, it became apparent that the service providers did not need to provide full 96 subscriber line capacity to small businesses and buildings. For the most part, 24 subscriber line capacity sufficed. Accordingly, expensive TR-8 RT equipment installed by these service providers on customer premises was under-utilized. Yet, the TR-8 interface standard specifies that each telecommunications exchange contain an interface module providing for at least one 96 channel RT connection, thus implying that each customer premises be equipped with TR-8 RT compatible equipment at an unacceptable cost of thousands of dollars.
Accordingly, it would be advantageous to provide a system to more economically provide telecommunications line access to remote subscribers without modifying the existing TR-8 interface modules within telecommunications exchanges.